Vacuum ion-getter pump with cryogenically cooled cathode

ABSTRACT

A vacuum ion-getter pump includes a vacuum chamber having a pumping port, an anode positioned in the vacuum chamber, a cathode positioned in the vacuum chamber in proximity to the anode, a voltage source coupled between the anode and the cathode, a magnet assembly to produce a magnetic field in the vacuum chamber, and a cooling device thermally coupled to the cathode. The cooling device may be a cryogenic cooling device.

FIELD OF THE INVENTION

This invention relates to vacuum pumps known as vacuum ion-getter pumpsand, more particularly, to vacuum ion-getter pumps having cooledcathodes for improved performance. Vacuum ion-getter pumps are sometimesreferred to as sputter ion pumps.

BACKGROUND OF THE INVENTION

The basic structure of a vacuum ion-getter pump includes an anode, acathode, and a magnet. The anode includes one or more pump cells, whichmay be cylindrical. Cathode plates, typically titanium, are positionedon opposite ends of the pump cells. A magnet assembly produces amagnetic field oriented along the axis of the anode. A voltage,typically 3 kV to 9 kV, applied between the cathode plates and theanode, produces an electric field which causes electrons to be emittedfrom the cathode. The magnetic field produces long, more or less helicalelectron trajectories. The relatively long trajectories of the electronsbefore reaching the anode improves the chances of collision with gasmolecules inside the pump cells. When an electron collides with a gasmolecule, it tends to liberate another electron from the molecule,forming a positive ion. The positive ions travel to the cathode due tothe action of the electric field. The collision with the solid surfaceproduces a phenomenon called sputtering, i.e., ejection of titaniumatoms from the cathode surface. Some of the ionized molecules or atomsimpact the cathode surface with sufficient force to penetrate the solidand to remain buried.

Prior art vacuum ion-getter pumps have generally satisfactoryperformance, but exhibit certain limitations. Such pumps have limitedpumping capacity for light gases, such as hydrogen and helium. Inaddition, such pumps require a starting pressure on the order of 10⁻² to10⁻³ torr in order to begin operation.

U.S. Pat. No. 5,357,760, issued Oct. 25, 1994 to Higham, discloses aso-called hybrid cryogenic vacuum pump wherein a separate cryopump and aseparate ion-getter pump are positioned in one vacuum chamber. Thedisclosed vacuum pump does not overcome the limitations described above.

Accordingly, there is a need for improved vacuum ion-getter pumps andmethods for operating vacuum ion-getter pumps.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a vacuum ion-getter pumpcomprises a vacuum chamber having a pumping port, an anode positioned inthe vacuum chamber, a cathode positioned in the vacuum chamber inproximity to the anode, a voltage source coupled between the anode andthe cathode, a magnet assembly to produce a magnetic field in the vacuumchamber, and a cooling device thermally coupled to the cathode.

The cooling device may be a cryogenic cooling device, such as a closedcycle refrigerator. The closed cycle refrigerator may have a cold headin the thermal contact with the cathode. The anode may be operated atroom temperature or may be cooled.

According to a second aspect of the invention, a method is provided foroperating a vacuum ion-getter pump of the type including an anode and acathode positioned in a vacuum chamber. The method comprises cooling thecathode. The cathode may be cryogenically cooled. The method may furthercomprise coupling the vacuum chamber to an enclosure to be evacuated,applying a voltage between the anode and the cathode and producing amagnetic field in the vacuum chamber.

According to a third aspect of the invention, a vacuum ion-getter pumpcomprises a vacuum chamber having a pumping port, an anode positioned inthe vacuum chamber, a cathode positioned in the vacuum chamber, and acryogenic cooling device thermally coupled to the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a schematic diagram of a prior art ion pump cell;

FIG. 2 is a schematic diagram of a prior art vacuum ion-getter pump; and

FIG. 3 is a simplified schematic diagram of a vacuum ion-getter pump inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A schematic diagram of a prior art ion pump cell is shown in FIG. 1. Acylindrical anode cell 20 has a cell axis 22. Anode cell may befabricated of stainless steel, for example. Cathode plates 24 and 26 arepositioned at opposite ends of anode cell 20 and may be perpendicular tocell axis 22. A power supply 30 applies a voltage, typically 3 kV to 9kV, between the cathode plates 24, 26 and the anode cell 20. A magnetassembly (not shown in FIG. 1) produces a magnetic field 32 in anodecell 20 parallel to cell axis 22.

A schematic diagram of a prior art vacuum ion-getter pump havingmultiple anode cells is shown in FIG. 2. Like elements in FIGS. 1 and 2have the same reference numerals. The ion-getter pump of FIG. 2 includesmultiple anode cells 20 a, 20 b, . . . 20 n located between cathodeplates 24 and 26. Power supply 30 is connected between cathode plates24, 26 and anode cells 20 a, 20 b, . . . 20 n.

A magnet assembly 40 includes magnets 42 and 44 located on opposite endsof anode cells 20 a, 20 b, . . . 20 n. Magnet 42 may have a north polefacing anode cells 20 a, 20 b, . . . 20 n, and magnet 44 may have asouth pole facing anode cells 20 a, 20 b, . . . 20 n. A magnet yoke 50of magnetic material provides a return path for magnetic fields betweenmagnets 42 and 44. In the configuration of FIG. 2, magnetic yoke 50 hasa generally rectangular shape. In other prior art ion-getter pumps, themagnet yoke may be U-shaped, with an open side. Magnets 42 and 44produce magnetic field 32 in the region of anode cells 20 a, 20 b, . . .20 n. The entire assembly shown in FIG. 2 may be enclosed in a vacuumchamber.

The voltage between cathode plates 24, 26 and anode cells 20 a, 20 b, .. . 20 n results in the generation of free electrons in the anode cellvolume. These free electrons ionize gas molecules that enter the anodecells. The ionized gas molecules are accelerated to the cathode plates,usually made of titanium or tantalum, resulting in sputtering of thecathode material onto surfaces of the anode cells. The sputtered cathodematerial readily pumps gas molecules and is the primary pumpingmechanism in the ion pump. Secondary electrons produced from theionization process sustain the plasma in the anode cells so that thepumping action is continuous. The magnetic field axial to the anodecells is required to maintain a long electron path and to sustain astable plasma in the anode cells.

A simplified schematic diagram of a vacuum ion-getter pump in accordancewith an embodiment of the invention is shown in FIG. 3. The pumpincludes an anode 120 and a cathode 122. Anode 120 includes anode cells120 a and 120 b in the embodiment of FIG. 3. Cathode 122 includescathode plates 124 and 126, and end plate 128 in the embodiment of FIG.3. Anode cells 120 a and 120 b are located between and are spaced fromcathode plates 124 and 126. End plate 128 is connected between cathodeplates 124 and 126. The ion pump may include one or more anode cells.Each anode cell may have a cylindrical configuration and may befabricated of stainless steel. The anode cells 120 a, 120 b, areoriented with their axes parallel to each other and perpendicular tocathode plates 124, 126. Cathode plates 124 and 126 and end plate 128may be fabricated of titanium or tantalum, for example, or othersuitable metals or alloys.

A power supply 130 applies a voltage, typically 3 kV to 9 kV, betweencathode 122 and anode 120, and more particularly between cathode plates124, 126 and anode cells 120 a, 120 b. Cathode plates 124 and 126 areelectrically connected together, and anode cells 120 a and 120 b areelectrically connected together.

A magnet assembly 140 provides a static magnetic field 142 in the regionof anode cells 120 a, 120 b to facilitate vacuum ion pumping. In theembodiment of FIG. 3, magnet assembly 140 includes magnets 144, 146, 148and 150, each of which may be a permanent magnet. It will be understoodthat different magnet arrangements may be utilized within the scope ofthe invention.

Anode cells 120 a and 120 b, cathode plates 124, 126 and end plate 128are positioned with a vacuum chamber 160. Vacuum chamber 160 is sealedvacuum-tight, except for a pumping port 162 configured for attachment toan enclosure to be vacuum pumped. In the embodiment of FIG. 3, magnets140, 146, 148 and 150 are located outside vacuum chamber 160. In otherembodiments, the magnets may be located within vacuum chamber 160.

The cathode 122 is cooled, preferably cryogenically cooled, so as tocapture gas molecules by a combination of condensation, sorption andphysical burial of accelerated ions. As shown in FIG. 3, cathode 122 isthermally coupled to a cooling device 180. Cooling device 180 may be acryogenic cooling device, such as a closed cycle refrigerator. Cathode122 may be thermally anchored to a cold head 182 of a closed cyclerefrigerator. Cooling lines and other connections between cooling device180 and cold head 182 are isolated from the interior of vacuum chamber160.

One suitable refrigerator is based on the Gifford-McMahon cycle. It willbe understood that other refrigerator types, including other cryogenicrefrigerators, may be used within the scope of the invention. Therefrigerator preferably produces temperatures in the range used incryogenic vacuum pumps, but cooled cathodes operating at temperaturesabove the range used in cryogenic vacuum pumps have a positive effect onpumping performance.

As described above, the cathode 122 is cooled and is preferablycryogenically cooled. In other embodiments, anode 120 is also cooled andmay be cryogenically cooled. In the embodiment of FIG. 3, cold head 182may be thermally coupled to anode cells 120 a and 120 b, as indicatedschematically by dashed line 190.

In the vacuum ion-getter pump of FIG. 3, gas is pumped by capturingmolecules through different mechanisms. One mechanism includescondensation of gas onto the cold cathode surfaces. Other mechanisms arebased on creation of ions, confined by the magnetic field 142, that areaccelerated into the cathode where they are captured by: (a) chemicalcombination on the cathode surface forming stable compounds (mainlyoxides and nitrides); (b) burial and diffusion of small atoms, such ashydrogen, into the bulk of the cathode; (c) burial of noble gas atoms inthe cathode; and (d) more complex molecules, such as water, carbondioxide and methane, are dissociated in the high voltage discharge andtheir components are pumped by the above mechanisms.

Advantages of the disclosed pumping scheme include: (1) increasedhydrogen pumping capacity due to the low temperature of the cathode, (2)the ability to pump from high starting pressures, and (3) the ability topump light gases at temperatures well above those of a typical cryogenicpump operating at 20K.

Sievert's law describes the relationship between:

-   -   P=equilibrium pressure of hydrogen in torr;    -   Q=concentration of hydrogen in solid solution in the metal        cathode in torr-liters/gram;    -   T=temperature in Kelvin;    -   A, B=coefficients related to the cathode metal.

Sievert's law is stated as:

P=A+2 log Q−B/T

Solving for concentration Q gives

$Q = {\sqrt{\frac{P}{10^{A}}}10^{\frac{B}{2T}}}$

As the temperature goes down, the equilibrium concentration of hydrogenat a given pressure goes up. This fact is well established and isutilized in getter pumps.

Cryocondensation of common gases, such as nitrogen, oxygen, carbondioxide and water, onto the cryogenic cathode provides the ion pump ofthe present invention the ability to pump at pressures above thestarting limit of the typical vacuum ion pump. When the total pressureis below the vacuum ion pump starting pressure, typically 10⁻² torr, ionpumping begins and gases which do not condense at higher temperaturesare captured.

The vacuum ion-getter pump of the present invention can capture lightgases, such as helium, hydrogen and neon, at a base temperature abovethat of a typical cryogenic pump. This reduces the thermal load on theclosed cycle refrigerator and decreases the refrigerator's requiredcapacity.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A vacuum ion-getter pump comprising: a vacuum chamber having apumping port; an anode positioned in the vacuum chamber; a cathodepositioned in the vacuum chamber in proximity to the anode; a voltagesource coupled to between the anode and cathode; a magnet assembly toproduce a magnetic field in the vacuum chamber; and a cooling devicethermally coupled to the cathode.
 2. The vacuum ion-getter pump asdefined in claim 1, where in the cooling device comprises a cryogeniccooling device.
 3. The vacuum ion-getter pump as defined in claim 2,wherein the cryogenic cooling device comprises a closed cyclerefrigerator having a cold head in thermal contact with the cathode. 4.The vacuum ion-getter pump as defined in claim 2, wherein the coolingdevice operates at temperatures used in cryogenic vacuum pumps.
 5. Thevacuum ion-getter pump as defined in claim 3, wherein the cryogeniccooling device is based on the Gifford-McMahon cycle.
 6. The vacuumion-getter pump as defined in claim 1, wherein the cooling devicecomprises a cryogenic refrigerator.
 7. The vacuum ion-getter pump asdefined in claim 2, wherein the cathode comprises spaced-apart cathodeplates and wherein the anode comprises a plurality of anode cellspositioned between the cathode plates.
 8. The vacuum ion-getter pump asdefined in claim 2, wherein the magnet assembly comprises permanentmagnets positioned outside the vacuum chamber.
 9. The vacuum ion-getterpump as defined in claim 2, wherein the anode operates at or near roomtemperature.
 10. The vacuum ion-getter pump as defined in claim 2,wherein the anode is thermally coupled to a cryogenic cooling device.11. The vacuum ion-getter pump as defined in claim 2, wherein thevoltage source maintains a voltage in a range of 3 to 9 kilovoltsbetween the anode and the cathode.
 12. A method for operating a vacuumion-getter pump of the type including an anode and a cathode positionedin a vacuum chamber, the method comprising: cooling the cathode.
 13. Themethod as defined in claim 12, wherein cooling the cathode comprisescryogenically cooling the cathode.
 14. The method as defined in claim12, wherein cooling the cathode comprises operating the cathode attemperatures used in cryogenic vacuum pumps.
 15. The method as definedin claim 13, further comprising operating the anode at room temperature.16. The method as defined in claim 13, further comprising cooling theanode.
 17. The method as defined in claim 12, further comprising:coupling the vacuum chamber to an enclosure to be evacuated; applying avoltage between the anode and the cathode; and producing a magneticfield in the vacuum chamber.
 18. A vacuum ion-getter pump comprising: avacuum chamber having a pumping port; an anode positioned in the vacuumchamber; a cathode positioned in the vacuum chamber; and a cryogeniccooling device thermally coupled to the cathode.
 19. The vacuumion-getter pump as defined in claim 18, further comprising a magnet toproduce a magnetic field in the vacuum chamber.